1. Field of the Invention
The invention relates to a powertrain for a hybrid electric vehicle with front and rear driving axles that are mechanically independent.
2. Background Art
It is known design practice for a hybrid electric vehicle powertrain to use traction controlled systems and stability control systems for the vehicle. These separate systems usually are interfaced in a powertrain assembly that cannot independently control each driving axle. More recent design concepts for hybrid electric vehicle powertrains use a mechanically independent electric motor drive for one of the axles as the other axle is powered by an internal combustion engine. Examples of hybrid electric vehicle powertrains with that configuration are disclosed in U.S. patent publications US 2002/0041167 and US 2005/0178592. A vehicle chassis dynamics control system for such hybrid electric vehicle powertrains cannot independently request the powertrain to adjust (increase or decrease) driving torque for one set of traction wheels and to separately adjust torque for the other set of traction wheels.
If a traditional interface for the separate sets of traction wheels is used in a hybrid electric vehicle powertrain with two sets of traction wheels, it may be necessary to apply friction braking at a slipping axle in order to achieve traction control if a powertrain controller does not reduce traction wheel driving torque sufficiently to avoid wheel slipping. Application of friction braking to a traction wheel that has minimal traction will oppose torque distribution to one set of traction wheels thereby wasting energy and degrading acceleration performance for the vehicle.
Unlike a hybrid electric vehicle with two sets of traction wheels and a center differential that provides a mechanical connection between the driving axles for the traction wheels, a hybrid electric vehicle with independent driving axles does not include a center differential assembly for maintaining good acceleration performance by redirecting driving torque from an axle with slipping traction wheels to the axle that has better traction. In a HEV with separate front and rear powertrains and no center differential, therefore, application of friction braking alone cannot redirect torque to the axle with better traction available. Further, if friction brakes are applied to the traction wheels that have low traction, the brakes may have a tendency to degrade both the stability of the control system and acceleration performance.